1. Field of the Invention
The present invention relates to wax hydrocracking and, more specifically, to hydrodewaxing using shape-selective zeolites.
2. Description of the Prior Art
As evidenced by the patent and scientific literature, it is well known that crystalline zeolite catalyst are widely used in various hydrocarbon conversion processes. Crystalline aluminosilicates have been found to be particularly effective for a wide variety of hydrocarbon conversion processes and have been described and claimed in many patents, including U.S. Pat. Nos. 3,140,249; 3,140,252; 3,140,251; 3,140,253; and 3,271,418. Aside from being general catalysts and hydrocarbon conversion processes, it is also known that the molecular sieve properties of zeolites can be utilized to preferentially convert one molecular species from a mixture of the same with other species.
In a process of this type, a zeolite molecular sieve is employed having catalytic activity within its internal pore structure and pore openings such that one component of a feed is capable of entering within the internal pore structure thereof and being converted to the substantial exclusion of another component that, because of its size, is incapable of entering within the pores of the zeolitic material. Such shape-selective catalytic conversion is also known in the art and is disclosed and claimed in U.S. Pat. Nos. 3,140,322; 3,379,640; and 3,395,094.
Recently, attention has focused on a novel class of catalysts useful in the dewaxing of gas oils, lube base stocks, kerosenes, and whole crudes, including syncrudes obtained from shale, tar sands, and coal hydrogenation. U.S. Pat. No. 3,700,585 discloses the use of ZSM-5 zeolite to efficiently catalyze dewaxing of various petroleum feedstocks. U.S. Pat. No. 3,700,585 discloses and claims the cracking and hydrocracking of paraffinic materials from various hydrocarbon feedstocks. The patent is based upon work on the dewaxing of gas oils, particularly virgin gas oils, and crudes, although its disclosure and claims are applicable to the dewaxing of any mixture of straight chain, slightly branched chain, and other configuration hydrocarbons. The catalyst may have a hydrogenation/dehydrogenation component incorporated therein. Other U.S. Patents teaching dewaxing of various petroleum stocks are U.S. Pat. Nos. Re. 28,398; 3,852,189; 3,191,540; 3,894,933; 3,894,938; 3,894,939; 3,926,782; 3,956,102; 3,968,024; 3,985,050; 4,067,797; and 4,192,734.
Catalytic hydrodewaxing can be considered a relatively mild, shape-selective cracking or hydrocracking process. It is shape-selective because of the inherent constraints of the catalyst pore size upon the molecular configurations that are converted. It is mild because the conversion of gas oil feed to lower boiling range products is limited, e.g., usually below about 35%, and more usually below about 25%. It is operative over a wide temperature range but is usually carried out at relatively low temperatures, e.g., start of run temperatures of from about 270.degree. C. are usual. Generally speaking, shape-selective catalytic hydrodewaxing is usually conducted in a single stage, "single stage" meaning that the dewaxing is customarily conducted in one large reactor, or in several reactors in series, with no intermediate heating, cooling, removal of impurities, etc., between reactor beds. This is to be contrasted to conventional hydrocracking processes, which usually operate in several stages with one or more quench stages to prevent temperature runaway.
In U.S. Pat. No. 4,446,007, there is disclosed a shape-selective catalytic hydrodewaxing process wherein the reaction temperature is raised relatively rapidly to at least about 360.degree. C. after start-up. While the process disclosed in U.S. Pat. No. 4,446,007 (incorporated herein by reference for all purposes) gives an optimum start-up, it does not provide optimum operation of the process thereafter; i.e., while the rapid start-up procedure makes the dewaxing unit an efficient generator of high octane gasoline during start-up, it does not solve the problem of working the catalyst to the maximum extent possible or extending the run length. The latter problem was solved by the processes disclosed in U.S. Pat. Nos. 4,935,120 and 4,994,170.
In the process disclosed in U.S. Pat. No. 4,935,120 (incorporated herein by reference for all purposes), hydrocracking the wax was accomplished in at least a first stage reaction zone and at least a second stage reaction zone, the effluent from the first stage being heated and charged to the second stage reaction zone.
In the process disclosed in U.S. Pat. No. 4,994,170 (incorporated herein by reference for all purposes), hydrocracking the wax was accomplished in two stages, with the first stage containing at least 20 wt % of the dewaxing catalyst and the second stage containing at least 20 wt % of the dewaxing catalyst, there being added at least a portion of the hydrogen downstream of the first stage, the total hydrogen to the second stage being greater than the total hydrogen to the first stage.
It is known that certain catalysts are useful in "hydrotreating processes" wherein a hydrocarbon feed is contacted with the catalyst in the presence of hydrogen and under selected conditions to remove heteroatoms such as sulfur, nitrogen, oxygen, and metallic contaminants such as nickel, vanadium, and iron from the feed and/or to saturate aromatic hydrocarbons and/or olefinic hydrocarbons in the feedstock and/or to hydrocrack the feedstock. For example, U.S. Pat. No. 4,113,656 (incorporated herein by reference for all purposes) discloses hydrotreating catalysts comprising at least one Group VIB metal component and at least Group VIII metal component that are ideally suited for such hydrotreating processes. U.S. Pat. No. 3,954,671 (incorporated herein by reference for all purposes) discloses a hydrotreating catalyst comprising a co-gelled composition comprising a crystalline zeolitic molecular sieve component containing less than 5 wt % sodium and containing ions selected from Mn, rare earths of atomic numbers 58-71 and alkaline earths, the catalyst further comprising an alumina-containing gel component, a Group VI hydrogenating component, and a Group VIII hydrogenating component.